Ethylene vinyl alcohol copolymer (EVOH) is known to provide an excellent oxygen barrier property when used in making packaging films. And while films containing EVOH have been developed and are being used, the physical properties inherent to EVOH lend certain difficulties to the fabrication and use of EVOH-containing films. EVOH is stiff and punctures easily. EVOH is not readily thermoformed. Orientation of EVOH is accomplished only with difficulty, and within a narrow range of process operating parameters.
These problems are recognized and addressed in U.S. Pat. No. 4,347,332 NYLON/EVOH BLENDS, issued Aug. 31, 1982; and U.S. patent application Ser. No. 290,172 entitled POLYMERIC BLENDS OF EVOH COPOLYMER AND FILMS MADE FROM THOSE BLENDS, filed on Sept. 20, 1982 now abandoned, in the name of David L. Newsome. Another related U.S. patent application is Ser. No. 464,730 entitled ORIENTED EVOH/NYLON BLEND FILM, which is a continuing U.S. patent application of Ser. No. 290,171, filed Aug. 5, 1981 now U.S. Pat. No. 4,911,112. The above documents teach improvements in technology relating to EVOH-containing films. And while the improvements do represent advances in technical knowledge, there remain certain problems associated with the use of EVOH polymers.
It is also known that EVOH is susceptible to degradation of its oxygen barrier property in the presence of high humidity, or moisture vapor content, at the EVOH layer. In order to ensure that the EVOH has a low humidity environment within which to operate, the EVOH is usually used in combination with other polymeric layers which are positioned so as to provide a moisture barrier between the EVOH layer and an environment potentially having an unacceptably high moisture content. Thus it is common to find EVOH used in multiple layer structures which include moisture barrier materials such as polyethylenes.
It is further known that EVOH loses some of its sensitivity to moisture, and thus has improved functionality as a barrier component of film, when it is molecularly oriented or otherwise formed without remelting after the making of a film containing EVOH. The process of orienting the EVOH, however, has proved to be difficult because of the physical properties of EVOH. In general, the EVOH, either alone, or in a multiple layer film, has a tendency to split or tear during the orientation process. Although the mechanism is not known for sure, this tendency might be explained in terms of the high degree of crystallinity of EVOH films. Whatever the mechanism, processing of EVOH subsequent to forming a film has proved to be extremely difficult. This difficulty has retarded large scale use of EVOH in the oriented form. While some oriented films may be made which are functionally acceptable, many of those films exhibit a high haze level, which is commercially unacceptable.
One method of improving the processability of EVOH film is taught by U.S. Pat. No. 4,451,599, issued May 29, 1984. In that patent, it is taught that certain plasticizers may be blended into the EVOH to improve its performance in the orientation process. While these plasticizers do fulfill the plasticizing role, in that the plasticized EVOH film can be subsequently oriented with ease and the expected reduced moisture sensitivity can be achieved, the plasticizers migrate toward the surface of the film and change the film surface properties over a period of time. While the surface properties may not be critical for some uses, in many cases the changes in surface properties caused by the migration are not acceptable, and another means of modifying the properties of the EVOH layer is desired.
Another method of improving properties of an EVOH film is set forth in U.S. Pat. No. 4,347,332. In that patent, it is proposed to blend nylon along with a plasticizer, with the EVOH in order to make a clear and desirable film. While a clear and desirable film may be made by this process, it has been found that, after a relatively short processing time in the extrusion of some of the polymer blends to form film, gels begin to appear in the film and get progressively worse until shut-down of the extrusion process is required.
The critical issue addressed herein by the inventor, then, is that of modifying the EVOH to avoid its undesirable physical characteristics; so that the EVOH layer is susceptible to commercial processes of extrusion and orientation, to produce a commercially acceptable clear barrier film. The perplexing problem is that of discovering a compound or family of compounds which can be blended with EVOH to form a compatible blend and which will also modify the undesirable properties; all this without suffering the problems of migration and surface property changes commonly associated with plasticizers.
It is an object of this invention to provide a novel composition of modified EVOH which can be extruded continuously for long periods of time to produce novel films tougher than similar films having unmodified EVOH.
It is another object of the invention that the novel films be easily susceptible to orientation in the machine direction--uniaxially--and in both the machine direction and cross-machine direction--biaxially.
It is further object that the resulting oriented film be clear, that is, exhibit a low level of haze. The low haze in the oriented form, then, confirms the functional compatibility of the materials, and, in turn, good layer integrity, while satisfying the appearance requirements essential to its commercial acceptance. The films should exhibit good oxygen barrier properties as exemplary of EVOH, and improved oxygen barrier properties at high humidity, as compared to those in the unoriented form, as exemplary of oriented EVOH.
It is yet another object that the physical properties of the film surface, and particularly surface friction, be more or less constant with time.